1. Field of the Invention
The present invention relates to a control device of an internal combustion engine.
2. Description of Related Art
An internal combustion engine such as a diesel engine that basically performs lean combustion at an air-fuel ratio higher than the theoretical air-fuel ratio frequently switches combustion to rich combustion at an air-fuel ratio equal to or lower than the theoretical air-fuel ratio. For example, in the case where an exhaust pipe is equipped with a NOx catalyst (LNT: a lean NOx trap) to purify nitrogen oxides (NOx) discharged from the engine, the NOx is occluded to the LNT under a lean atmosphere. Then, after an interval, the atmosphere is switched to a rich atmosphere to reduce the occluded NOx to harmless nitrogen. Also in the case of the lean combustion as the basic combustion, there is a possibility that the air-fuel ratio deviates from the target to the rich side when intake system control cannot be performed suitably during acceleration or the like, for example.
When the combustion is switched from the lean combustion to the rich combustion, an opening degree of an intake throttle is reduced and a valve for exhaust gas recirculation (EGR) is opened, thereby suddenly changing intake system targets such as an air excess ratio. In this case, intake system controlled variables such as the air excess ratio do not necessarily follow the targets immediately. Rather, there is a transition period, so ignition timing can largely deviate from an adjusted state. If the ignition timing deviates largely from the adjusted state to the advanced side of the optimum ignition timing, the combustion progresses excessively, thereby causing problems such as a noise and excessive torque, for example. If the ignition timing deviates largely to the delayed side of the optimum ignition timing, for example, problems such as a misfire, increase of unburned fuel in the exhaust gas or insufficient torque can occur. The above phenomena can occur also when the deviation of the air-fuel ratio occurs during the acceleration.
With regard to this problem, Patent document 1 (JP-A-2005-48724) describes a technology that calculates present injection timing by interpolating the injection timing in the rich combustion and the injection timing in the lean combustion in accordance with the air excess ratio of every moment in a transient state where the combustion switches from the lean combustion to the rich combustion. Patent document 1 states that the appropriate injection timing can be calculated by the above method to cause the ignition timing to follow the target value.
FIG. 6 shows an example of temporal transitions of (a) the opening degree of the intake throttle, (b) the opening degree of the EGR valve, (c) the air excess ratio, (d) an injection timing command value and (e) the ignition timing of the technology of Patent document 1. The combustion is switched from the lean combustion to the rich combustion at time t0 as shown in FIG. 6. As the combustion is switched from the lean combustion to the rich combustion, the intake throttle opening degree is decreased as shown in part (a) of FIG. 6 and the EGR valve opening degree is increased as shown in part (b) of FIG. 6. Thus, the intake air is restricted and the recirculated exhaust flow rate increases, so the air excess ratio decreases gradually as shown in part (c) of FIG. 6.
With the method of Patent document 1, the injection timing command value of every moment is calculated as shown in part (d). It is known that the rich combustion has a high tendency to cause incomplete combustion, thereby increasing emission of soot from the engine. Therefore, in some cases, the target ignition timing is delayed in the rich combustion to provide a sufficient time from the injection timing to the ignition timing such that the fuel is sufficiently mixed with the air and the incomplete combustion is avoided. The target value of the ignition timing is set on the delayed side also in the example of FIG. 6. However, in the example of the conventional technology shown in FIG. 6, the actual ignition timing shown by a solid line in part (e) is not delayed fully to the target value shown by a broken line in part (e).
It is regarded that the problem shown in FIG. 6 is caused because the command value of the injection timing in the transient state is not calculated appropriately. The technology of Patent document 1 calculates the fuel injection timing in the transient state by interpolating the fuel injection timing in the lean combustion and the fuel injection timing in the rich combustion using only the air excess ratio. However, the information of the air excess ratio is not the only factor related to the calculation of the optimum fuel injection timing. According to knowledge of the inventors, it is necessary to take following factors into account in order to accurately calculate the appropriate fuel injection timing in the situation where the in-cylinder oxygen quantity and the like change suddenly.
First, it should be taken into account that the ignition delay differs between the case where the ignition timing is early and the case where the ignition timing is late. Generally, there is a characteristic that the ignition delay lengthens as the ignition timing becomes more distant from the top dead center. As stated above, generally, the ignition delay is set longer in the rich combustion than in the lean combustion to mix the fuel and the air sufficiently and to suppress the emission of the soot. Accordingly, the control is performed such that the ignition timing differs between the rich combustion period and the lean combustion period. If it is not taken into account that the ignition delay also differs because of the difference in the ignition timing, suitable injection timing cannot be obtained.
Moreover, the ignition delay is affected by existence/nonexistence of pilot injection, injection quantity of the pilot injection and injection timing of the pilot injection. The pilot injection is an injection for injecting small quantity of the fuel before main injection. Thus, the pilot injection has an effect of promoting mixing of the fuel and the air before the ignition. If the mixing of the fuel and the air is promoted, the incomplete combustion is inhibited, so the emission of the soot is suppressed. Generally, there is a characteristic that the ignition delay shortens as the pilot injection quantity increases and/or the pilot injection timing comes closer to the main injection.
Thus, the ignition delay increases/decreases in accordance with the ignition timing and the pilot injection. Therefore, unless such the increase/decrease in the ignition delay is taken into account, suitable injection timing cannot be calculated in the state where the in-cylinder oxygen quantity changes suddenly. It can be said that the technology of Patent document 1 merely calculates the injection timing by simply regarding that the ignition delay is constant, without taking the above factors into account.